This invention relates to a thin film semiconductor device for display and a method of producing same, and in particular to a thin film semiconductor device for display used in a large active matrix liquid crystal display with a built-in peripheral driving part and a method of producing same.
First, a common construction of an active matrix liquid crystal display device will be briefly explained, with reference to FIG. 1. As shown in FIG. 1, this active matrix liquid display device has a flat panel structure comprising a main substrate (101), a opposed substrate (102) and a spacer (103) affixing the main substrate (101) to the opposed substrate (102), and liquid crystal is held between the two substrates. On the surface of the main substrate are formed a display part (106) consisting of pixel electrodes (104) and switching devices (105) for driving the pixel electrodes (104) arranged in a matrix, and peripheral driving parts (107) connected to the display part (106). The switching devices (105) consist of thin film transistors. Thin film transistors are also formed in the peripheral driving parts (107) as circuit elements. The main substrate (101) having the above-described constitution will hereinafter be called a "thin film semiconductor device for display".
In the field of thin film transistors (TFT) integratedly formed in thin film semiconductor devices for display, structures using semiconductor films made from polysilicon are now being developed intensively and have been applied to relatively small (a few inches) active matrix liquid display devices. However, because polysilicon TFTs are made by high temperature processes, quartz substrates, which have excellent heat resistance, have been used for them. On the other hand, for comparatively large active matrix liquid crystal display panels (ten or so inches to several tens of inches) quartz substrates are not suitable due to their high cost and consequently glass substrates are employed. When a glass substrate is used, because its heat resistance is inferior, amorphous silicon TFTs, which can be made by relatively low temperature processes, have been employed. However, amorphous silicon TFTs have low mobility and P-channel amorphous silicon TFTs cannot be made. As a result it is impossible to form a peripheral driving part on the glass substrate, and an externally attached driver device is used and mounted by the TAB method or the like. Consequently the number of pixels is restricted by the size of the screen and mounting limits. There therefore is a limit on how high-density thin film semiconductor devices for display using amorphous silicon TFTs can be made. Moreover, because an amorphous silicon TFT has low mobility, to obtain a sufficient ON-current the transistor size inevitably becomes large. Consequently the area of the display part occupied by the amorphous silicon TFTs for switching becomes large, and this is a hindrance to the realization of a high aperture ratio.
Recently, polysilicon TFTs with high mobilities which can be produced by low temperature processes have been being intensively developed. This technology involves converting an amorphous silicon film into a polysilicon film by locally heating the amorphous silicon film by annealing using an excimer laser. However, it is difficult for the processes other than the forming of the semiconductor films to be made low temperature processes and made performable with large substrates, and consequently this technology has not reached practical application. For example, one process which becomes problematic is that of forming the gate insulating layer. The gate insulating layers of current polysilicon TFTs are produced by thermally oxidizing polysilicon at approximately 1000.degree. C. When the above thermal oxidation is replaced by some other method in which the film-forming process is carried out at a low temperature, the gate insulating film lacks a sufficient withstand voltage. Also, in order to build the peripheral driving devices in, to simultaneously build N-channel TFTs and P-channel TFTS, ion implanting of an impurity has been carried out, but an ion implantation apparatus which can handle large substrates has not been realized, and difficult problems arise. Plasma vapor phase diffusion apparatuses for use in place of ion implantation apparatuses are now being developed, but impurity control is difficult and their practical use for mass production has not been realized. In addition to the above, the most difficult problem is that it has not been possible to produce TFTs having an LDD structure (hereinafter called LDD-TFTs) by low temperature processes and without using ion implantation. LDD-TFTs are indispensable as thin film transistors for switching, and are employed in small active matrix liquid crystal display devices to prevent pixel leakage. However, it is extremely difficult at present to form LDD-TFTs by low temperature processes and without using ion implantation.